Sa Prefab Bridges 02

COMMISSION 6: PREFABRICATION TASK GROUP 6.3. PRECAST BRIDGES

STATE OF THE ART REPORT PRECAST BRIDGES

COMMISSION 6: PREFABRICATION TASK GROUP 6.3. PRECAST BRIDGES

STATE OF THE ART REPORT: PRECAST BRIDGES INDEX:

1. 2. 3. 4.

5. 6.

7.

8.

SCOPE DEFINITIONS HISTORY. PAST EXPERIENCES TYPOLOGY. PRESENT TECHNOLOGY. 4.1. ELEMENTS 4.2. ISOSTATIC BRIDGES 4.3. PARTIAL CONTINUITY BRIDGES 4.4. STRUCTURAL CONTINUITY BRIDGES 4.5. INTEGRAL BRIDGES 4.6. SEGMENTAL BRIDGES 4.7. SPECIAL BRIDGES 4.8. CABLE STAYED BRIDGES AESTHETICS DESIGN 6.1. SPECIFIC ASPECTS 6.2. DETAILS 6.3. DURABILITY 6.4. SEISMIC ASPECTS EXECUTION 7.1. PRODUCTION PROCESS 7.2. QUALITY CONTROL 7.3. TRANSPORT AND ERECTION 7.4. SITE WORK 7.5. TESTING 7.6. MAINTENANCE, INSPECTION AND ASSESMENT BIBLIOGRAPHY AND PICTURE CREDITS

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1. Scope The intention of this paper is to provide a description of the present status of development of solutions for precast concrete bridges, of which there is now an astounding variety. The document also attempts to focus attention on other areas where specific research is needed to clarify real or fictitious problems arising in the development of such structures. The development of precast concrete bridges actually has rather a long history, with the first designs dating from practically the initiation of prefabrication itself. Solutions dating from the thirties may be found in most developed countries, although admittedly for short span bridges and generally restricted to small works. It was not until the fifties that these structural solutions began to develop more intensely. The vigorous development that took place from that time on was largely driven by advances in the ways and means available for transport and erection the precast members as well as by progress in research on prestressing with steel wires and strands. While in some countries the Ministries of Transport or equivalent bodies draft codes on standard series of prefabricated bridges, generally limiting such regulations to the transversal sections and leaving manufacturers free to determine tendon placement and dimensioning, most countries have not proceeded in this manner, allowing precast concrete manufacturers to develop their own solutions. At present the prevailing approach is to refrain from regulating these members, leaving the development of new types of solutions to manufacturers' imagination and research. Development has been continuous, speedy and quite spectacular since the fifties. The three basic reasons for the success of prefabricated bridges are as follows: - Speedy construction - High cost-competitiveness compared to cast-in-situ solutions - Minimum disturbance of traffic in the event of bridges to be built over railways or roads. The primarily beam - solutions initially developed for these structural members were fiercely criticised because of their obvious aesthetic limitations, the inherent difficulties in reasonably adapting them to curved bridges and the problems ensuing from the large number of joints, which caused user discomfort as well as maintenance problems. Other criticism levelled at prefabricated bridges at the time was less justified and falls within the usual repertoire of objections to prefabricated structures. This criticism hinged essentially on two issues: - The idea that prefabricated structures are monotonous and ugly. - Although not generally expressed clearly and explicitly, rather a large number of designers offered considerable resistance to prefabrication, and in particular to prefabricated bridges, because they saw this type of industry to be competition that would reduce the demand for the services of engineering firms engaging in design. This latter objection, veiled and unexpressed, has been latent in numerous countries for a good many years. Certain specific technical issues were also referred, such as the initial doubts that arose with regard to the performance of prestressing tendon transfer length, particularly in the case of tendons with large diameters subjected to fatigue stress - typically in the case of railroad bridges - or about the effectiveness of the shear strength at the interface between the cast-insitu concrete and the prefabricated members, also especially in the event of fatigue stress. The prefabricating industry's response to these problems has been speedy, spectacular and conclusive. STATE OF THE ART REPORT PRECAST BRIDGES

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It has responded to the accusation that its structures are monotonous and ugly by creating an extraordinarily wide variety of forms, often with greater freedom than in cast-in-situ bridges, developing members with variable depths, solving the problem of curved plan bridges and providing for hyperstatic continuity which not only confers structural advantages but eliminates the need for joints, thereby precluding maintenance and discomfort issues. Surfaces, edges and surface textures that are impossible to achieve via cast-in-situ execution are within reach in prefabricated construction. The visual appearance of the concrete in precast members, moreover, is more apt to be of a high quality than in cast-in-situ solutions. The veiled criticism based on the fear of losing design work has been eradicated by the power of reality that always underlies practical evidence. Moreover, the development of cranes, launching cradles and assembly methods in general, along with the increased length and loading capacity of transport vehicles, have enhanced prefabrication potential enormously, both from the technical (longer spans) and financial standpoints. All this has led to the existence of prefabricated structures that can readily bridge spans of 100 m; and, as discussed below, in the light of the combination of these advantages and other modern techniques, there is no appreciable difference between the potential of precast and cast-in-situ concrete. Indeed, solutions involving a precast deck, generally comprising large members, and a stayed bridge system, used not only on the grounds of its structural effectiveness but as an evolutive system, have eliminated the specific span limitations affecting prefabrication and increased its competitiveness enormously. The truly beneficial advances in recent years in transversal deck constitution using strut bracing and precast members in general have made it possible to build very wide decks with lightweight as well as highly aesthetic monotube beams. All the forgoing has substantially enhanced the three essential advantages listed at the beginning of this section. The prefabrication industry has, in a period of barely 50 years, proved able to respond to important challenges, solving many problems and creating technology that has led to solutions that are: - of greater technological excellence - more cost-competitive - of higher aesthetic quality - faster to build.

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2. Definitions Bridge: Civil engineering construction works mainly intended to carry loads related to comunication over a natural obstacle or a communication line. This includes all types of bridges, especially road bridges, footbridges, railway bridges, etc. Abutment: Any end support of a bridge usually without rigid continuity with the deck. Rigid abutments and flexible abutments should be distinguished where relevant. Pier: Intermediate support of a bridge, situated under the deck. Bearing: Structural device located between the deck and an abutment or pier of the bridge and transferring loads from the deck to the abutment or pier. Prestress: Permanent effect due to controlled forces and/or controlled deformations imposed on a structure. Various types of prestress shall be distinguished from each other as relevant (for example prestress by tendons, prestress by imposed deformation at supports). Headroom: Free height available for traffic. Continuous bridge: Bridge with no expansion joints between adjacent intermediate spans, with or without structural continuity Integral bridge: Bridge with no expansions joints - neither between adjacent intermediate spans nor between end spans and abutments Diaphragm: Transverse deck stiffening beam of insitu or precast concrete construction Crosshead: Transverse support beam at an intermediate deck support Sagging moment: Bending moment inducing tension in the bottom fibres (positive moment) Hogging moment: Bending moment inducing tension in the top fibres (negative moment) Internal bonded tendon: Post-tensioning tendon contained within a ribbed ducting which is fully filled with a cementitious grout.

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3. History. Past Experiences. 3.1. Germany The first prestressed concrete bridge built with prefabricated beams was executed in Germany in 1938 by WAYSS & FREYTAG. The underpass at Oelde in Westfalen crossed the Autobahn without intermediate piers. It was realised only two years after the beginning of the construction of prestressed concrete bridges. The length of the I-shaped concrete beams was 33 M. They were cast on site in a bed and then moved into their final position. 1938: First prestressed concrete bridge „Underpass at Oelde in Westfalen„ (WAYSS & FREYTAG)

According to Freyssinet the beams were cast in two phases: The lower flange was cast first and then posttensioned, then the web together with the upper flange was cast. The hardening process was accelerated by steam curing. [3.1] Nearly two decades later the industrialised production of prestressed precast beams for bridges started in West-Germany, initiated by the Federal Railway Administration. The need for shorter erection times and for the least hindering of traffic led to the design of the first standardised railway-crossings. Two main production methods were developed for the railway-bridges: HOCHTIEF used prestressed precast I-beams, weight up to 8,0 tons. After positioning cross posttensioning cables in the upper flange and in the ribs an orthotropic slab was created. The joints were cast in situ (wet-joints). DYCHERHOFF & WIDMANN used a „contact“ system. The precast elements were box girders with smooth side walls. The cross posttensioning cables in the lower and in the upper flange realised by pure contact the orthotropic system (dry-joints). In East-Germany several standardised precast beams, mainly hollow-box-sections, were developed for railway and road-bridges, all were posttensioned in the transverse direction. Later elements with sufficient torsion stiffness were used and transverse posttensioning could be omitted. The advantage of the high degree of shop precasting brought some disadvantages in the construction of road-bridges: * inaccuracies could be corrected only within the screeding, which was unsatisfactory for the demanded high comfort required for the road surface, * the isolation-layer could not always be placed accurately, *many joints produced problems for maintenance (icing), *continuos bridges could not be constructed. In the 1960’s precast bridges without transverse posttensioning were designed as so-called „mixed systems“, e.g. precast beams with structural in-situ concrete on top of the beams and in-situ concrete for cross beams at support. This method allowed for tolerances, no transverse posttensioning was required and the orthotropic behaviour was easily achieved. Several variants of these „mixed systems“ developped: *beams placed close to each other. The in-situ concrete slab had no joints. (Classical mixed system). * precast beams placed with gaps in between and in-situ concrete slab without joints. * precast beams placed with smal gaps and in-situ parts in between, which take bending moments. *precast beams placed with smal gaps and in-situ parts in between for transfer of shearforces (beam-rows). *precast beams with in-situ slab in between * precast beams and precast slabs with in-situ topping. STATE OF THE ART REPORT PRECAST BRIDGES

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In 1979 guidelines for design and execution of bridges with precast beams were published [3.2]. The guidelines include design rules, examples and details specific for precast bridges. These recommendations are valid still to-day and only some minor amendments had to be made due to the progress of the state of the art. The latest developments in the construction of precast bridges include continuous bridges which are prestressed for transportation and after placing continuously posttensioned. Up to 200 m length (up to 7 spans) were continuously posttensioned.

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4. Typology. Present Technology 4.1.

Elements

4.1.1 - Decks having precast beams as main resistent elements. Some types of precast beams used in bridges are sketched in fig. 4.1: - square beams (fig.4.1 a) - I beams (fig. 4.1 b, c, ) - T beams (fig. 4.1 d) - inverted T beams (fig. 4.1 e, 4.1f) - U beams, V beams (fig. 4.1 g, h) Cast in situ concrete

a)

b)

c)

d)

e)

f)

g)

h)

Figure 4.1.1 Bridge elements.

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The following sub types of decks result:

Cast in situ slab

Precast slab (or formwork)

Figure 4.1.2 Bridge Decks. Precast beams, completed by a cast in place concrete topping

Figure 4.1.3 Isostatic precast I beams bridge in Spain.

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In–situ slab

Precast beam

a)

In–situ slab

Precast beam

b)

Figure 4.1.4 One or more U beams, completed by cast in place topping

Figure 4.1.5 Precast Box beams bridge in Spain.

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connections

view cast in-situ slab

precast beam

cross section

Fig. 4.1.6. segmental beams, completed by cast in place topping

Figure 4.1.7 Variable depth box beams bridge in Spain.

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a)

Transverse reinforcement

Precast elements

Transverse reinforcement

b)

Precast elements

Fig 4.1.8. Infilled precast beams

Fig 4.1.9. Precast box elements without topping

Fig 4.1.10. Precast box elements without topping in Holand STATE OF THE ART REPORT PRECAST BRIDGES

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4.1.2. - Solid slabs Decks formed by precast slabs on the entire span, having longitudinal shear keys, completed by cast on site topping or postensioned transversally. Solid slabs are usually used on minor spans.

Figure 4.1.11 – Partially precast slabs 4.1.3. - Precast piers and crosshead. 4.1.3.1. Piers and crossheads in portal frame - Solid crosshead - Hollow crosshead - Crosshead formed by two walls

Figure 4.1.12 Precast Piers and I beam in Spain.

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4.1.3.2. Single pier 1. Box beam with single support - Prismatic - Prism trunk - With column head 2. Box beam with double support - Hammer - Palm or/and Y - Solid prismatic block - Frame

Figure 4.1.13 Precast single pier and box beams in Spain. 3. With hollow crosshead, for beam double T, launder or caisson - Single pier - Dowels joined in height

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Figure 4.1.14 Precast single Pier for I beams in Spain. 4.1.3.3. Pile pier, raymond piles

Cast on site foundation

Figure 4.1.15 – Precast piers and crosshead 4.1.4. - Precast abutments In some cases precast elements are used to form bridge abutments. The elements are full height, modular width, and usually have, on the buried side, one or more webs from the top to the foundation. The resisting section is thus shaped as a T or π. For mayor heights a precast STATE OF THE ART REPORT PRECAST BRIDGES

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tie can be used to form a truss structure. The elements are placed on site near each other and are completed by cast on site foundations and a top beam.

Cast in situ top beam Precast abutment

Cast in situ footing

Figure 4.1.16 – Precast abutments

Figure 4.1.17. Precast Abutment and wall in Spain. 4.1.5. - Precast arches or vaults The vaults are of curved or poligonal guideline structures of different types: Domed structures with more than two elements. These have an arch supported on two curved side walls that are fixed to a raft foundation or directly supported on the ground with precast strip footings incorporated to the precast piece. Domed structures with two elements which are composed of two curved lateral walls. Domed structures with one element, with the bottom slab built into the precast or in-situ element. According to its functional scheme, they can be divided in several types: I.- An arch simply supported on two side walls connected by a bottom slab. II.- An arch simply supported on two side walls supported on two isolated footings. STATE OF THE ART REPORT PRECAST BRIDGES

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III.- Domed structure with two elements, commonly known as triarticulated. The connection to the foundations is a simple articulation; the foundations can be a bedplate or a footing . IV.- Domed structure of an element which can be fixed to the foundations or biarticulated. The different elements are usually of reinforced concrete, with rectangular section or ribbed section. The aspect from inside the passage is usually plain, although there may be solutions in which the lateral walls are ribbed inside (particularly in the second type of structure with large clearances). The arches generally have a rectangular section, although for large spans (L>10 metres) are usually ribbed. In the case of vaults with two elements, the support points may be raised with some walls to give more clearance. Multi-arch passages may be achieved by making two vaults share the same foot or wall

Wing wall

Precast collar wall

Wing wall

Precast concrete Arch elements

Continuous cast in place concrete footing

Figure 4.1.18 – Example of precast concrete arch structure

Figure 4.1.19 Precast arch structure in Spain. STATE OF THE ART REPORT PRECAST BRIDGES

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4.2.

Isostatic bridges

This chapter is mainly concerned with bridge decks constructed using factory made prestressed concrete bridge beams and designed isostatically with simple supports. The beams are generally used as follows:a) Beam and slab decks – prestressed beams positioned at centres and connected with a cast in-situ reinforced concrete deck slab. b) Solid infill decks – prestressed beams positioned contiguously and encased in cast insitu concrete such that a solid deck is formed. Transverse reinforcement is positioned through holes in the webs of the prestressed beams. The in-situ concrete generally covers the tops of the beams by approximately 75mm allowing the positioning of a top mesh cover down. Beam and slab decks are popular in the span range of 20 – 40m and solid infill decks are popular in the smaller span range of less than 20m. From the early days of prestressed bridge beam manufacture it was considered logical to design bridge decks as simply supported and with end joints. Joints were incorporated between intermediate spans and between end spans and abutments. Beams would be erected onto individual bearings – one at each beam end – and the joints would be dimensioned to allow thermal movement of the concrete within the bridge decks. Isostatic bridge design also suited the nature of prestressed concrete bridge beams in that deformities due to creep , shrinkage and temperature could occur in relative isolation. In the same way, differential settlement of deck supports was easily accommodated. Many thousands of bridges have been built as described, and the bridge beams have proved highly durable. The main reasons for the high durability are as follows: - The consistent use of high strength, low water/cement ratio concrete. - The prestress design providing an absence of cracking under working loads. - An almost guaranteed provision of specified concrete cover to secondary reinforcement (links). Although the beams themselves have proved highly successful, there are disadvantages inherent in simply supported deck design. Bearings, as previously stated, are generally required at each end of each beam. They are costly and eventually have to be replaced. However, the main problem with simply supported deck design is that of the joints. This problem became apparent following the practice of using de-icing salt on the road/bridge surfacing in winter. The salt solution was then able to penetrate the joints in the bridge decks and corrode the bearings and the concrete support structure. Good detailing at the tops of piers and abutments can help delay the onset of this corrosion. The following are examples: - The provision of means of inspection (and replacement) of bearings. - The provision of drainage channels for removal of water (fig. 4.2.1). - The provision of access/inspection galleries at abutments (fig. 4.2.2).

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However, it was realised that the best solution to de-icing salt penetration would be the elimination of all joints within the bridge deck. The elimination of joints between intermediate decks would require continuous bridges. The elimination of joints between the intermediate decks and also between end spans and abutments would require integral bridges.

Fig 4.2.1 – Drainage channel

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180 0 10 00

Fig 4.2.2 – Access gallery

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4.3.

Partial continuity bridges

Partial continuity is a method of providing continuity between intermediate bridge decks such that no distribution of vertical load effects between the intermediate bridge decks occurs. This applies to all vertical loads – dead, superimposed dead and live. Two methods of providing partial continuity in beam and slab decks will be described. A third method, providing ‘notional’ continuity is also described.

Typical features: 1. 2. 3.

Separate bearings and diaphragms are provided for each span Deck slab is separated from support beams for a short length to provide rotational flexibility There is no continuity reinforcement between ends of beams and there is no moment continuity between spans

Figure 4.3.1 – Partial Continuity detail type 1 – Continuous separate slabs This continuity detail confines itself to the deck slab only, which flexes to accommodate the rotations of the simply supported deck beams. The beams are erected in the conventional manner onto individual bearings. To permit this flexure, the deck slab is separated from the support beams for a length of about 1.5m by a layer of compressible material. In-situ reinforced concrete diaphragms may be located at the ends of the separated slab.

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Typical features: 1. The tie reinforcement at mid-depth of the slab is debonded for a short length either side of the joint to permit deck rotation. There is no moment continuity between spans. 2. Slabs between spans are separated using compressible joint fillers but deck waterproofing and deck surfacing are continuous and special seals are provided over the joint for double protection. 3. Separate bearings and end diaphragms are provided for each span.

Figure 4.3.2 – Continuity detail type 2 – Tied deck slab The bridge decks are designed and constructed in the conventional multispan simply supported manner with slab trimmer diaphragms at the beams ends. As with type 1, the beam ends are carried on two parallel rows of bearings on the piers. Long connecting reinforcement dowels are incorporated at the slab mid-depth to tie the slabs together over the pier, eliminating expansion movement at deck level and permitting the use of a buried deck rotation joint. To accommodate this rotation, the dowels are debonded and sleeved from the surrounding slab concrete over short lengths either side of the joint. Also, the slab and trimmer beam downstands are “necked” using compressible joint filler below and above the dowel connection. Types 1 and 2 continuity are the logical extension to providing simply supported decks with continuity. They offer the minimum of extra design and construction effort.

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4.4.

Structural continuity bridges

Typical features: 1. 2. 3.

Beams are erected on temporary supports generally off pier foundations Permanent bearings are in single line Continuity reinforcement is provided in the slab and at the top and bottom of bridge beams. The lapping of reinforcement is normally not difficult.

Figure 4.4.1 – Continuity detail type 3 – Wide in situ integral crosshead This type of continuity detail uses prestressed beams significantly shorter than the spans between support piers. The beams are usually supported on temporary trestles built off the pier foundations. The wide in-situ integral crosshead over the pier is then cast between and around the beams to provide about 1m embedment. Longitudinal continuity is accomplished by reinforcement within the continuous composite deck slab, generally supplemented by reinforcement, and pretensioning strand extending from the top and bottom of the embedded beams. Transverse strength is provided by either prestressing tendons or reinforcement, some of which may pass through holes in the ends of the precast beams. The crosshead is supported on a single row of bearings set centrally on the pier. Although more complex to design and more expensive to construct than any of the other methods, type 3 continuity offers more advantages. Plan curvature can be readily accommodated by varying the width of the integral crosshead to form a plan trapezium shape. This permits the use of a standard length beam per span. Vertical curvature problems can be reduced by vertically curving the top and bottom surfaces of the crosshead. This reduces the increased slab thickness at midspan required to take up the vertical curvature above the straight chorded precast beams. A single central row of bearings is required. This immediately halves the number of bearings required for simply supported construction, although individual bearing size will increase. Piers are thinner, not only because a single line of bearings takes up less room at the pier top, but because the dead and live load moments applied to the piers by off centre pairs of bearings are removed.

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Full width piers are not required. The integral crosshead can be designed to allow considerable deck cantilevering outside the pier. This also provides a further reduction in the number of bearings.

Typical features: 1. 2. 3.

Temporary supports are not required Permanent bearings may be in single or twin line Continuity reinforcement is provided in the slab and at the bottom of bridge beams The lapping of reinforcement is difficult.

Figure 4.4.2 – Continuity detail type 4 – Narrow in situ integral crosshead The prestressed beams are long enough to be erected onto two parallel rows of temporary or permanent bearings on the pier tops. The in-situ integral crosshead over the pier is then cast between and around the beams to provide about 1m embedment. The crosshead is, however, narrower than type 1 because of the small gap between the beams. This same narrow gap makes adequate bottom flange reinforcement connection difficult between beams. Longitudinal hogging bending continuity is again readily established by top reinforcement within and extending well into the continuous composite deck slab. Transverse strength of the crosshead is generally provided for by reinforcement, some of which passes through holes in the ends of the beams. Where twin rows of temporary bearings are used, a central row of permanent bearings located under the crosshead is brought into use by removing the temporary bearings after the crosshead concrete has gained sufficient strength. Some examples use a wide single permanent rubber bearing which acts as a seating for both beams. This type of continuity is relatively easier to construct than type 3 but cannot offer the advantages of extra span or curvature. The greatest advantage lies in the ease of beam erection directly onto the pier bearings. Nevertheless, adequate connection between bottom flange reinforcement is difficult.

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Typical features: 1. 2. 3. 4.

Beams are supported on stage 1 crosshead during erection. Crosshead to be monolithic with pier Crosshead soffit is normally lower than beam soffit Reinforcement is similar to types 1 and 2 depending on the cross-section of the stage 1 crosshead.

Figure 4.4.3 – Continuity detail type 5 – Integral crosshead cast in two stages This type of continuity detail is a variant of types 3 and 4 where the integral crosshead is cast in two stages. The crosshead is of greater depth than the main deck beams and the bottom section is cast first to support these beams, generally on thin mortar beds. The second stage cast completes the integral crosshead in the manner described for type 3. The advantage of this type of continuity is the complete elimination of bearings. The disadvantage is that the downstand half of the crosshead is obstructive, both aesthetically and in terms of headroom. Type 5 continuity is also described as framed construction in that the piers are monolithic with the deck and consequently contribute with the moment distribution.

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Figure 4.4.4 Continuous precast rectangular beams bridge in Germany.

Figure 4.4.5 Detail of Continuity of precast rectangular beams bridge in Germany.

Figure 4.4.6. Detail of Continuity of precast rectangular beams bridge in Germany.

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Figure 4.4.7. Continuous precast rectangular beams bridge in Germany.

Figure 4.4.8. Continuous precast rectangular beams bridge in Germany.

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Figure 4.4.9. Continuous precast I beams bridge in France.

Figure 4.4.10 Continuous precast I beams bridge in France.

Figure 4.4.11 Continuous precast I beams bridge in France.

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Figure 4.4.12 Continuous precast Box beams bridge and Piers in Spain.

Figure 4.4.13 Continuous precast Box beams bridge and Piers in Spain.

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Figure 4.4.14 Continuous precast Box beams bridge and Piers in Spain.

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4.5.

Integral bridges

Integral bridges are designed with no expansion joints – neither between adjacent intermediate spans nor between end spans and abutments. Chapters 5 and 6 have described various methods of deck continuity – whether partial or structural. This chapter describes the various types of integral abutments and associated design issues. In the same way that bridge decks can be continuous over intermediate supports but still require bearings, it is possible for there to be no expansion joints at abutments and bearings still be provided. This type of construction is referred to as semi-integral and is particularly suited to prestressed beam bridges in that the bearings eliminate the problems associated with moment continuity and rotation due to creep and thermal effects. Abutments of integral bridges are attached to the bridge, and so have to move horizontally in response to temperature fluctuations in the bridge. The abutments must be designed to allow this movement to occur, at the same time as being able to resist longitudinal traffic loads. The design of integral abutments involves different considerations from the design of conventional fixed abutments. Similarly, the design of integral abutments for bridges built using prestressed concrete bridge beams also requires special considerations which do not arise in integral bridges using other forms of construction. Various types of integral and semi-integral abutments are shown in Figure 4.5.1 Some countries recommend a limit to the overall length and skew of bridges designed integrally. (U.K. – maximum overall length = 60m, maximum skew = 30°). In America, integral bridges have been designed and constructed successfully to overall lengths in excess of 200m.

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....

Figure 4.5.1 – Integral abutments

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4.6.

Segmental bridges

A segmental deck is a structure formed by a number of precast segments having a length of the same order of magnitude as the depth of the deck, connected together by joints transverse to the direction of the span (fig. 4.6.1). The performance of the joints between elements influences the way the structure works, there being discontinuity in the passive reinforcement.

Figure 4.6.1 – Precast segment 4.6.1 Joints The joints between segments may be provided in three ways: Mortar joints: a mortar joint is provided with a width of several centimetres. Glued joints: joints where, before closure, a layer of epoxy or other synthetic resin is applied on the surface. Dry joints: joints where there is no material between the segments in contact. Such type of joint is not covered by this Standard. There are some conditioning factors related to the use of each type of joint: a) In case of mortar joints, the postensioning cannot be applied until the mortar has reached sufficient strength; b) Glued joints require that adjacent concrete surface match.This is generally achieved by using as mould the surface of the adjacent segment (method of match casting). 4.6.2 Keys The joint between segments has to be capable to transmit forces parallel to its plan: shear and torsion. In order to increase the capacity of load transmission salient shear keys can be provided. The keys may be of a large size in small quantities or multiple and of a small size (the latter is preferred). The single keys are designed as short brackets in order to transmit the total shear, while the multiple keys are not reinforced and the shear transmission capacity is checked using an enhanced coefficient of friction.

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4.7.

Special bridges

Besides normal precast bridges with beams, there are also other possibilities for bridges, like arch, external postensioning or the use of struts for long span bridges.

Figure 4.7.1 Precast Arch bridge in Spain.

Figure 4.7.2 Precast bridge with external postensioning in Spain.

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Figure 4.7.3 Precast bridge with side cantilevers and single pier in Spain.

Figure 4.7.4 Precast bridge with inclined struts.

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Figure 4.7.5 Precast bridge with external postensioning.

Figure 4.7.6 Precast “Wing” bridge

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Figure 4.7.7 Precast variable depth elements bridge

Figure 4.7.8 Precast curved beams bridge

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4.8.

Cable stayed bridges

It is possible to use stays to reach longer spans in the construction of precast bridges. Spans up to 400 metres can be achieved by building with precast decks in cable stayed bridges. Precast decks can be designed for two planes of stays with a box girder under each plane of stays. It is also possible to design a deck with a single plane of stays with one or two box girders joined by a transverse beam at each anchorage of the stays. For shorter spans, up to 120 metres, other kind of bridges can be designed. Bridges supported with extradosed prestressing or with additional struts.

Figure 4.8.1 Precast cable stayed bridge in Germany.

Figure 4.8.2 Precast cable stayed bridge in Germany. Detail.

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Figure 4.8.3 Centenario Precast cable stayed bridge in Spain.

Figure 4.8.3 Centenario Precast cable stayed bridge in Spain. Detail.

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5.

Aesthetics

References should be made mainly to fib-Guide to good practice (bulletin No. 9, Guidance to good bridge design, July 2000), chapter 1.4.2. [5.1] Amendments are required for some specific aspects of precast bridges: Decisions in favour of or against a design with precast elements should not be based only on technical and economical reasons. They result very often from prejudices and a thorough investigation proves frequently that the differences in pure construction cost are only small. Average figures are nearly identical for precast and in-situ structures: as an example one can assume that for a bridge with slenderness l/h ≈ 20 the following values per m² are to be taken into account: * Concrete: 0,50 m³, Prestressing Steel: 15 kg, Steel S500: 70 kg. * Cost for transport and erection comply mostly with cost for scaffolding. Economical advantages can be achieved when other aspects are taken into account: * existing traffic (road or rail) which does not allow the placing of scaffolding at the required area and which would require additional costly measures like moving or lifting of structure into final position * erection of new structures for existing traffic which allows only very short interruptions * unfavourable soil conditions in between the abutments * reduction of construction time * construction is rather independent from unfavourable weather conditions * economical advantages due to rate of capacity utilisation at the manufacturer The only seeming disadvantage of precast girder bridge decks is the fact that the surface is often considerably larger than for an in-situ design. However this is compensated by substantial higher density and quality of the outer concrete shell which leads to a better durability and a longer expected service life. The options for aesthetic detailing are a little bit more limited when precast elements are used. [5.1]: The rules for proportions, transparency, slenderness, unity and harmony are valid for all bridge structures be they in-situ or precast. There are however many possibilities to influence the design in a positive manner: * Careful attention must be given to the design of details and secondary elements. The slenderness con be accented by painting the rim of the corbels in a bright warm colour and leaving the web of the beams in a dark colour. * Other alternatives are shown in slides No. 14 and 15. There are examples where the outer edge beam became special shape to accent the slenderness The layout of the beams can nowadays easily be curved in order to follow the gradient of the road. Special attention must be given to some aspects in the design and erection of these beams, but by solving these problems allows precast bridges to be more competitive in use. The intermediate support of continuous precast bridges is an important problem which must be handled with special attention. Its functional detailing leads to unpleasant and clumsy details at bearings and at the intersection between main beam and crossbeam. In urban areas acoustic barriers are often added to the bridge. This sound absorption walls can dominate the appearance of the structure. Therefore it is essential to know in time of these STATE OF THE ART REPORT PRECAST BRIDGES

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measures. The detailing of the structure can only be successful and sensible if the final view is taken into consideration. Example of shaped edge beam: (principle):

painted with bright colour

Fig 5.1 Precast bridge in Spain

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Fig 5.2 Precast bridge in Spain

Fig 5.3 Precast bridge in France STATE OF THE ART REPORT PRECAST BRIDGES

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Fig 5.4 Precast bridge in France

Fig 5.5 Precast bridge in Germany

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Fig 5.6 Precast bridge in Holand.

Fig 5.7 Precast bridge in Holand.

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Fig 5.8 Precast bridge in Holand.

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6. Design 6.1.

Specific Aspects

The integral advantage of the combination precast + high quality materials require the clarification of many specific regulations. Between them should be appointed: * II.1. * II.2. * II.3. * II.4. * II.5.

Values of γc and γf to apply in the case of precast members. Contribution of the concrete to the strength to shear at the interface in function of the fatigue level. Influence of the form and position of the connection reinforcement for shear at the interface. Strength of compressed struts at shear, especially with HPC. Transfer length of tendons with HPC.

As in any other structural typology, the development of prefabricated bridges is hindered, in a way, by the need to solve problems via research, which delays the extension of the use of such solutions. In our case, attention should be drawn to the following. a) Obviously, in the case of the manufacture of bending members, the fundamental part of prefabricated bridges, prefabrication affords higher quality than cast-in-situ execution in many respects. - Cutting, bending and placing reinforcement bars and tendons. - Low pouring height together with very powerful concrete compaction procedures. - Extraordinarily efficient curing systems. All this means, in short, that the quality of the members, in particular bending members, is much higher in prefabrication than in most cases of cast-in-situ execution. Since, broadly speaking, under the deterministic approach the bending safety factor is expressed by the equation εs = kγ sγ f

where k is a coefficient that varies very little from one in the case of prestressing, the only way to establish a suitable procedure to reward the highest prefabrication quality is, obviously, to make it truly dependent on the load factor γf by means of the quality control to which the structure is subject. The idea frequently invoked to simply reduce the γc coefficient entails a more psychological than actual reward. The reduction of the γc coefficient, indeed, only rewards cases as columns or where compression on struts is limited in the event of shear, but in all other respects its importance for bending members is essentially negligible. It is only by reducing the γf coefficient that an appropriate balance between the different execution procedures can be re-established, at least as long as the present semi-probabilistic systems continue to be used. STATE OF THE ART REPORT PRECAST BRIDGES

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b) The limitation on compressed struts had evolved pessimistically in recent years in the Model Code as well as in the successive versions of the Eurocodes, both with respect to the regulations dealing with concrete in general and those relating to precast concrete in particular. Recent research (1) has shown that such reductions were unwarranted: Fortunately, new proposals to revise Eurocode EC-2 have re-established a more realistic solution. c) The anchoring conditions for prestressed tendons, especially in high strength concrete, need to be analysed in greater depth, not only to establish the transfer length but to provide for a more accurate evaluation of the distribution of stress throughout the transfer zone to improve shear stress studies in these areas of members.

The shear strength of conection with “in situ” slab is 25% to 33% higher in b) than in a) Fig. 6.1.1. d) The shear interface problem calls for supplementary research in two directions. On the one hand the use of normal strength cast-in-situ concrete in conjunction with high strength prefabricated members will become more and more frequent, which places special demands on engineering for shear at the interface. Secondly, the present forms of computing the contribution of the connection reinforcement between cast-in-situ and precast concrete take account of the ratio of the area of reinforcement only. However, the way these bars are bent and placed has a substantial effect on shear strength and should be lent more attention in future (Fig. 6.1.1)

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6.2.

Details

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6.3.

Durability

6.3.1. Introduction Durability is the capability of a structure, assembly or component to maintain minimum performance and adequate levels of stability and serviceability over at least a specified time under the influence of degradation factors with anticipated maintenance but without excessive unforeseen maintenance. The requirement of an adequately durable structure is met if, throughout its required life, a structure fulfils its function with respect to serviceability, strength and stability without significant loss of utility. Durability of concrete bridges is mainly governed by the quality of materials. The quality of the concrete, the bearings, the joints and the sealing of the surface are the essential parts which together determin the length of the service life. At present the codes do not explicitly ask for the definition of the required lifetime of a structure. Generally the overall durability, as defined above, depends of the intended use of the structure together with load specifications. In some countries maintenance programmes are set up for bridges. These programs form part of the requirements for durability. They are regulated in specific codes other than design codes and aim at the minimisation of maintenance during life time of the structures. Thorough knowledge of degradation mechanism in a structure which could develop to future failure is essential to secure serviceablity and to reduce cost due to developing faults. Durability may be affected both by direct actions and also by consequential indirect effects inherent in the performance of the structure (e.g. deformations, cracking, water absorption, etc.). The possible significance of both direct and indirect effects is to be considered. The load-bearing capacity of bridges can be violated by the degradation of concrete and reinforcement. Therefore bridges (and all other structures) must be designed in a way that the minimum safety level is secured during the intended service life despite degradation and ageing of materials. As long as systematic durability design methods are still missing and common rules are not yet available as design tools, the general approach is to observe border conditions to improve performance of the structure and its components to avoid unexpected and premature failure. These stipulations are met when specific design rules, special quality demands for materials, material compositions, working conditions, structural dimensions etc. are observed. Durability calculations which could result in the use of modified safety factors in combination with traditional mechanical design will be developed in the future [1]. Until these methods are available durability design has to ensure a high standard of all parameters which could influence service life of the structure in general. To ensure an adequately durable structure, the following inter-related factors are to be considered: * Degradation Factors * Influence Of Environmental Conditions * Composition, Properties And Performance Of The Materials * Shape Of Members And The Structural Detailing * Quality Of Workmanship, And Level Of Control * Particular Protective Measures * Maintenance During The Intended Life. STATE OF THE ART REPORT PRECAST BRIDGES

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6.3.2. Degradation Factors The following factors may have negative long-term effects on the load-bearing capacity of concrete structures: * corrosion due to chloride penetration * corrosion due to carbonation * mechanical abrasion * salt weathering * surface deterioration * frost attack For reinforced concrete, corrosion protection for any type of reinforcement is provided by taking care of the following aspects: Concrete Stress conditions: Compliance e.g. with the requirements contained in the clause 4.4.1 of the ENV 1992-1-1. Excessive compressive stresses in concrete may promote the formation of cracks. Steel stress conditions: Compliance e.g. with the requirements in the clause 4.4.2 of the ENV 1992-1-1 results in limitation of cracks under SLS. To avoid complex crack-width calculations the following simplification is often used: Concrete stresses calculated at the uncracked section must not exceed the value σb: σb ≤ 0,463 β 2WN (σb and βWN in MN/m²). In cross-sections in reinforced concrete (e.g. webs or base plates for transverse flexural loading) the transverse flexural-tensile stresses (calculated in accordance as uncracked section) may not exceed the values of σb either. 6.3.3. Influence of Environmental Conditions Environment, in this context, means those chemical and physical actions, to which the structure as a whole, the individual elements, and the concrete itself is exposed, and which results in effects not included in the loading conditions considered in structural design. Durability parameters: * depth of deterioration of concrete and corrosion of reinforcement * concrete cover * diameter of rebars * Factors to be taken into account: * strength of concrete * permeability of concrete * type of cement * curing method * type of reinforcemenet * structural dimensions For values of concrete cover and acceptable crack-width reference could be made to design codes e.g. ENV 1992-1-1. 6.3.4. Composition, Properties and Performance of Materials Material must comply with the requirements of appropriate international or national standards. The choice of materials takes account of the environmental conditions including any aggressive actions. These should be considered in conjunction with other factors such as STATE OF THE ART REPORT PRECAST BRIDGES

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design and detailing, standards of workmanship and construction, and intended maintenance regimes - to produce the required level of intended maintenance regimes - to produce the required level of performance for the structure throughout its service life. Standard values can generally be increased by an allowance (∆h) for tolerances, which for precast elements may be assumed in the range 0 ≤ ∆h ≤ 5 mm depending on the standard of workmanship and quality control. In some countries the values for concrete cover may be reduced when prefabrication in a factory is ensured. 6.3.5. Shape of Members and Structural Detailing Early in the design process, the effects and possible significance of the environmental conditions is to be considered in relation to the durability requirements. Reference should be made to the design criteria in point 3 above and to the requirements for concrete cover to reinforcement in point 4. Other factors to be considered in design and detailing, in order to achieve the required level of performance, should include the following aspects: the adoption of a structural form which will minimise the uptake of water or exposure to moisture. the size, shape and design details of exposed elements or structures should be such as to promote good drainage and to avoid run off or standing pools of water. Care should be taken to minimise any cracks that may collect or transfer water. In the presence of cracks across a complete section, additional protective measures (coated bars, coatings, etc.)may be necessary. for the drain of the deck slab surface, the sealing surface must be detailed carefully. attention in design and detailing, to the different aspects of indirect effects. Expansion joints between bridge decks can seriously effect the long term durability of the structure. They can lead to serious corrosion of the r.c. elements of the deck and substructure due to leakage of salt solution through the joints. Several methods of eliminating joints in continuous decks are acceptable. 6.3.6. Quality of Workmanship and Level of Control The standard of workmanship on site shall be such as to ensure that the required durability of the structure will be obtained. The combination of materials and procedures used in making, placing and curing the concrete shall be such as to achieve satisfactory resistance to aggressive media for both concrete and steel. During construction, adequate measures shall be taken, by means of supervision and quality control, to ensure that the required properties of the materials and standards of workmanship are achieved. Quality Control: An other important factor to ensure the obtaining of the required durability is the quality control in the different phases of the construction. 7. Protective Measures In addition to all the facts that must be considered to ensure the required durability, other protective measures should also be taken into account. Sealing: Joints between precast elements must be sealed. Tendons placed in sheaths or ducts in the concrete, couplers and anchorage device shall be protected against corrosion. STATE OF THE ART REPORT PRECAST BRIDGES

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Should the delay between stressing and grouting exceed the time permitted, then protection of the tendons shall continue until grouting takes place. Where temporary protection is provided, the material used shall have an approval document and shall not have a deleterious effect on the prestressing steel or on the cement grout. Written instructions shall be provided for the site or the works for the preparation and execution of the grouting. It frost is liable to occur, measures shall be taken to prevent freezing of water in sheaths which are not yet grouted. After a period of frost, the sheaths should be free from ice before grouting is started. Tendons may be protected by materials based on bitumen, epoxy resins, rubber, etc., provided that there are not detrimental effects on bond, fire resistance, and other essential properties. 6.3.8. Control and Maintenance of the Completed Structure. A planned control programme should specify the control measures (inspections) to be carried out in service where long term compliance with the basic requirements for the project is not adequately ensured. All the information required for the structure’s utilisation in service and its maintenance should be made available to the person who assumes responsibility for the complete structure.

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6.4.

Seismic Aspects

6.4.1.Objectives, requirements and confeormity criteria 6.4.1.1. Objectives With respect to bridges, national policies for prevention of accidents associated with major hazards such as earthquakes involve ensuring the acceptability of the risk of failures that would interrupt traffic. This acceptability is assessed in terms of the socio-economic repercussions of such failures. Emergency transport must be maintained with an appropriate degree of reliability after the design seismic event has occurred. The design seismic event depends on local seismicity and other contingencies. 6.4.1.2. Requirements National Authorities can define two fundamental requirements for different kinds of structures, depending on the possible consequences of their collapse. a) No-collapse requirement (ultimate limit state) After occurrence of the design seismic event, the bridge must retain its structural integrity and adequate residual load bearing capacity. It must be capable of accepting damage (intended to help dissipate energy) during the earthquake and be able to carry emergency traffic and be inspected and repaired as required. Formation of plastic hinges in the piers is generally acceptable, but is prohibited in the deck which must also be fitted with devices to prevent it coming off its supports under the effect of extreme seismic displacement. b)Damage limitation requirement (serviceability limit state) Seismic action with a high probability of occurrence during the design service lifetime of the bridge must cause nothing more than minor damage to the parts of the structure designed to help dissipate energy during the seismic event, and engender neither traffic restrictions nor a need for immediate repair. 6.4.1.3. Conformity criteria The criteria aimed at explicitly meeting the no-collapse requirement implicitly cover the damage limitation requirement. a) Intended seismic behaviour Bridges must be designed to be ductile (inelastic behaviour) or have limited ductility (essentially elastic behaviour) under their design seismic actions. For both economic and safety reasons it is preferable to design bridges to have ductile behaviour in zones of moderate to high seismicity. This is achieved by ensuring that plastic flexural hinges are formed (normally in the piers, and normally accessible for inspection and repair) to dissipate energy, or by using isolation systems. Tall piers are generally designed to remain in the elastic range. Generally speaking, the bridge deck must also remain in the elastic range. STATE OF THE ART REPORT PRECAST BRIDGES

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b) Capacity design The capacity design method involves ranking the energy-dissipation zones and is aimed at ensuring the formation of flexural plastic hinges in the piers. The shear strength of the plastic hinges and the shear and flexural strength of all other regions must be checked in order to avoid any risk of brittle failure and to guarantee the elastic behaviour of standard areas. The capacity design method is used for bridges with ductile behaviour. For bridges with limited ductility the method is not indispensable, and the conventional design process for sections under seismic loading can be used. c) Connections and displacement control Equivalent linear analysis methods must be capable of determining the displacements engendered by the design seismic action with an appropriate degree of accuracy. For members with plastic hinges (this does not concern bridge decks), the secant stiffness at the theoretical elastic limit is generally adopted. Connections between supporting members (piers) and supported members (deck) must be designed appropriately so as to ensure structural integrity and to prevent unseating as a result of extreme seismic displacement. To ensure satisfactory overall ductility, structural and non-structural detailing must ensure appropriate behaviour of the bridge and its component parts, and appropriate clearances must be provided to protect important or critical structural members. These clearances must correspond to the calculated total displacement under the effect of seismic conditions: dEd

= dE + dG ± dTs

where: dE is the design seismic displacement dG is the long-term displacement due to permanent and quasi-permanent actions (shrinkage and creep) dTs is the displacement due to thermal movement for a representative value, T s, of temperature variation considered to be appropriate for the combination with seismic effects. It can be assumed that dTs is 40% of the maximum thermal movement. Displacements due to second-order effects are to be added if they make a significant contribution. Protection must be provided against major impacts caused by exceptional pounding between members of major structural importance, by using ductile members or dampers. The clearance in these elements must be at least the displacement dEd. At connections in rail bridges, differential transverse displacements must be either prevented or limited to appropriate values in order to prevent derailments. 6.4.2. Design principles 6.4.2.1. General design of the project Consideration of seismic effects is important right at the conceptual design stage, even for regions of low seismicity.

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Choosing ductile behaviour is generally appropriate in areas of moderate to high seismicity. In this case, the following factors generally need to be taken into account: Choose piers and abutments to withstand seismic forces in the longitudinal and transverse directions. Greater deformability reduces the level of the design seismic action but increases movements at joints and deformable supports, which can induce substantial second-order effects. In the case of continuous-deck bridges, where the transversal stiffness of the abutments and the shorter piers adjacent to them is great relative to that of the other piers, it may be preferable to use deformable bearings or bearings sliding transversally on top of the abutments and short piers. The locations of energy–dissipation systems must be carefully chosen to ensure they are accessible for inspection and repair. With exceptionally long bridges or bridges on heterogeneous soils, it will be necessary to decide on the number and locations of intermediate expansion joints. With bridges spanning potentially active tectonic faults, the probable discontinuity of ground displacements will generally have to be assessed and taken into account by means of structural flexibility or by appropriate layout of expansion joints. 6.4.2.2. Bridge site Bridges should not be sited near (less than 500 m) an active fault where earthquakes are particularly violent and poorly represented by the regulatory spectra. The bridge site can be called into question if it is discovered that there is a risk of liquefaction of the subsoil. If the site cannot be changed, it is vital that the bridge be founded on soils of appropriate characteristics beneath the liquefiable soils, and that adequate drainage be provided. 6.4.2.3. Distribution of spans – Location of supports The location of supports and the distribution of spans must be based on classical analysis of the gap, taking account of the following particularities. Span uplift : If the end spans are short (0.5 to 0.6 times the length of adjacent spans) and cannot be made longer, the end crossbeams on the abutments can be ballasted, or a hold-down device can be provided. End spans should systematically be extended in the case of highly skewed bridges ( α(1°cycle) /3 , the test has to be stopped. Acceptance criteria The loading test is considered positive if the following conditions are verified. In the mid-span of the main structural elements the ratio of measured value of vertical displacements to the theoretical one is less than: 1.1 for prestressed concrete bridges; 1.15 for reinforced concrete bridges. The remaining value of the vertical displacement is acceptable, according to the above mentioned “Remaining quantity criteria” The cracking pattern of the critical zones of the bridge is in agreement with the standard specifications. 7.5.5. Dynamic loading test In some European countries like Spain , Czech Republic and Slovakia the execution of dynamic load tests in some kind of bridges is mandatory. In Spain is necessary to carry out these tests in all the bridges with a span of over 60 m and in all railway bridges. In Czech Republic and Slovakia the dynamic load tests are compulsory on railway bridges with a simple span over 50 m or on continuos bridges with an overall length of 80 m. On highways the dynamic test are mandatory on bridges with spans of over 45 m. Execution of the loading test In dynamic load tests is possible to excite the structure in different ways: With vehicles moving on the bridge, with the impact of falling loads , with special vibrators (hydraulic actuators), etc. In dynamic load tests using vehicles, the bridge is passed over at various speeds. Testing starts at a speed of about 10 km/h , at 50 km/h and at the maximum speed allowed. Very often in road bridges the natural frequency of the bridge is obtained exciting the structure with the jump of a vehicle over an obstacle in the form of a cylinder segment , 6 cm high , 300 cm long and cross – sectional chord being 50 cm long. Measurements during the load test In the loading dynamic test project the following parameters that will be measured during the test should be indicated: -The eigenfrequency (or natural frequency) of the bridge. -The logarithmic decrement of damping of the bridge , given by the formula: ν=1/n ln(A0 / An ) where A0 : the amplitud of bridge oscillation. An : the amplitud of bridge oscillation after n cicles. -Impact coefficient. It is defined as: φ = fdin / fsta STATE OF THE ART REPORT PRECAST BRIDGES

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where fdin : is the maximum dynamic behaviour (for example deflection) under a dynamic load. fsta : is the maximum static behaviour (for example deflection ) under a static load of the same value. Instrumentation of the dynamic loading test Usually the data adquisition system is formed by the next elements. -Displacements transducers. -Accelerometers. -Strain gauges. -Signal conditioning equipment. -Data adquisition board or unit. The response of the bridge structure to the dynamic load is measured in the middle of each span. The sampling rate of the measures must be higher than 200 samples per second. Acceptance criteria The logarithmic decrement of damping should vary between 0.03 and 0.12 The impact coefficient will be evaluated as follows: Low if φ < 1,10 . Medium if 1,10 < φ < 1,30 . High if φ > 1,30 . Bridge structures with relatively low stiffness and damping should be designed not to have fundamental and higher frequencies in the range between 3.5 and 4.5 Hz. Footbridges and other pedestrian structures should be designed with natural frequencies lower then 1,6 Hz or higher then 2,4 Hz. 7.5.6. Final report When loading test is finished a technical report is prepared (written) concerning the loading test itself. This technical report contains information concerning: a description of the bridge; general data concerning the loading test; the geometry and the characteristics of the bridge; the situation of the bridge before the loading test; the means for loading the bridge; the measured quantity; the measuring instruments; the comparison between the measured and the theoretical values of the quantity. the final conclusion about the behaviour of the bridge in the test.

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7.6. Maintenance, Inspection and Assessment 7.6.1. Maintenance, inspection The maintenance, inspection and assessment are important elements of preservation of value, seen from the economy as a whole. Regular inspections of bridges can detect possible defects which can be caused due to the accident or due to the long-term actions of chemicals or environmental effects. The chemicals and environmental effects can be partially predicted for each component of the structure, which has its life time. On the base of this knowledge it is possible to create maintenance plan for every part of the structure. The maintenance of structures can prevent their further deterioration. The inspection, assessment and upgrading of structures are engineering disciplines which are gaining significance both from a technical and economic point of view. The codification of assessment procedures for existing structures is representing a new field of development. New non-destructive methods which are developed are most suitable and efficient for the assessment of bridges and other concrete structures e.g. concerning corrosion of the reinforcement, detection of faults in concrete members, location of ducts and the detection of fractures of prestressing steel. 7.6.2. Valuation of existing structures New methods of evaluation, repair and strengthening which are presented yet seem to be accepted by custom. The reasons are to be found in the higher demands concerning the variability of bridges use, the limited durability of bridges and increased safety requirements. The people responsible for design, execution and use need design rules for durability. Insufficient job site quality control procedures and non-existing and/or insufficient maintenance and inspection during use need to be covered by Design Concepts for Durability, both for the design of new structures and for the repair of damaged structures. The codification of assessment procedures for existing structures is a new field of development. Chapter 8 of the draft ISO 2394, Reliability of Structures, is especially devoted to the assessment of existing structures and may by regarded as the first international model code in this field. The analysis and assessment of an existing structure follows the same basic principles as the assessment of a structure under design. In the context of this code, this means that probabilistic models should be produced for the basic variables and that mechanical models should be made for the loads and structural properties. ISO 2394 treats these problems within the framework of probabilistic methods. For application in everyday practice, the results of those methods need to be translated into standard load and resistance factor procedures. If a structure is to be reconstructed, an assessment must be mandatory. The dimensions and properties of the present structure must be known in order to be able to make an analysis of the reconstructed building. Many bridges have been designed for traffic loads that are much smaller than the present day traffic loads. Many engineers have the feeling that no further investigation is required if the bridge does not show signs of distress. This is a dangerous way of reasoning. It should be at least verified that the structure is of such a type that it indeed will show signs of distress in due time. STATE OF THE ART REPORT PRECAST BRIDGES

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When there is obvious damage to a structure, a structural appraisal is clearly required. The difficult point is to indicate the demarcation between ‘harmless deterioration’ and ‘deterioration demanding further investigation’. In cases where a structure has been loaded by some extreme load, e.g. earthquake or fire, there is a need for further investigation, even if this particular structure does not show any visible damage at first sight. A routine inspections according to the maintenance plan can be relatively simple, e.g. looking at appearance aspects only. If the result is within the predefined limits, the assessment is that the structural capacity is still sufficient. If the result is outside the predefined limits, a further investigation might follow, or it might be decided to repair the structure immediately. The highest priority needs for bridge inspection, for instance, are a global bridge evaluation, the rapid assessment of asphalt covered bridge decks, the evaluation of unknown foundations, the evaluation of prestressing steel in prestressed concrete, and integration of the nondestructive evaluation into bridge management systems. 7.6.3. Nondestructive methods The following NDT methods are most suitable and efficient for the assessment of bridges and other concrete structures. Corrosion of reinforcement, concrete cover and potential mapping The state of corrosion of the reinforcement over an extensive area is assessed using a computer-aided concrete cover measuring instrument. In addition, the potential monitoring method is used to determine the potential difference between the reinforcement and a reference electrode over an extensive area in a previously defined grid. In the areas found to have a particularly low potential difference, a chloride section of the concrete is additionally analyzed. By comparing these 3 parameters the areas in which there is already active corrosion or which are highly likely to develop corrosion can be localized. This method is applied especially to bridges after removing the old carriageway surfacing or the old waterproofing course. The condition for application of the potential monitoring method is that the concrete must possess sufficient conductivity. The concrete must be a conductive electrolyte. The question to be asked here is whether or not the concrete surface needs to be pre-wetted before performing the measurement. The concrete does not have to be pre-moistened if it is permanently in contact with the atmospheric air. The air humidity usually ensures a sufficient level of moisture in the void system of the concrete. Ultrasonics and impact echo One of the principal objectives in the development of NDT-CE (Non Destructive Testing in Civil Engineering) techniques is a reliable assessment of the integrity or the detection of defects in concrete members. Such inspections could in the past only be solved by means of radiography, for a concrete thickness less than 0,6 m, or by partially destructive methods. The Ultrasonic Pulse-Echo or Pulse-Velocity methods are the right tools for this type of detection. When the load capacity of a concrete unit is in doubt, it is desirable to know where air voids, gravel pockets and areas of low concrete strength are located exactly. Ultrasound measurements were used to identify areas of low quality concrete in columns more than 800 mm thick by correlating pulse velocity with the compressive strength of sample cores drilled. It is shown that grid measurements and 3D visualisation are essential tools to obtain an easyto-read picture of the interior of the columns. The Impact-Echo method is particularly suitable for determining the thickness of concrete units from just one surface, but also for localizing defective areas and compaction defects in the concrete. Location of ducts by Radar STATE OF THE ART REPORT PRECAST BRIDGES

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Defective areas in concrete are traceable by radar. The radar beam – an electromagnetic signal – penetrates the concrete and is reflected at the interface with the defective area. Not only defective areas but also the prestressing tendons which usually have a concrete cover of at least 100 mm can be localized by radar. The electromagnetic pulse is reflected by the reinforcement and the tendon. The backscattered signals are received and displayed in dependence on the position of the antenna. The graphic recording of the radar signal shows the tendons as typical reflection hyperbolas. The main advantage of radar compared to other conductive detectors is its capability to detect deeply embedded metal sheathings behind the normal reinforcement. fractures of prestressing steel: Remanent magnetism Before repairing a prestressed concrete structure the prestressing tendons should also be examined to check whether they are intact. Opening of the grouted prestressing tendons always involves the risk of closing them reliably in order to avoid an area of corrosion with anodic polarization. A grouted prestressing tendon should not be inspected by random sampling, but must be checked over its entire length, at least in the areas subjected to extreme loading, using a nondestructive method. A method based on remanent magnetism has been developed. The testing equipment has been continually improved and allows the detection of failures with high accuracy, it can be mounted easily and offers efficient application. The main field of repair strategies are related to the design of corrosion protection for ordinary reinforcement. New class of corrosion inhibitors for reinforced concrete structures has been developed. The inhibitor can be applied on the surface of the concrete structure by brush, spraying or flow coating and will penetrate deep into the concrete to protect the reinforcing bars against corrosion or to stop corrosion without any need for the concrete to be removed. The inhibitors are based on amino alcohols (Ama) or are aqueous mixtures of partly neutralised AMAs. Rehabilitation of poorly grouted tendons The localization of metal sheathings which are incompletely grouted using nondestructive testing methods is only possible by radiographic testing and subsequent and elaborate image processing. This method is not suitable to be used over an extensive area. Nevertheless, it is helpful to gain an insight into poorly grouted metal sheathings using a partially destructive method, i.e. by tapping and opening the metal sheathing and inspecting it with an endoscope. These inspections focus on areas in which the probability of defective parts is particularly high, for example at the anchorage points or at the high and low points of the tendons. The size of the void can then be determined at the tapping points by means of vacuum measurement and, using the vacuum, the void can be grouted with injection grouting resin or cement paste.

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7. Bibliography [1.1] [1.2] [1.3] [1.4]

ENV 1992-2 1996 Eurocode 2: Design of concrete structures Part 2: Concrete Bridges ENV 1998-2 1994 Eurocode 8: Design provisions for earthquake resistance of structures Part 2: Bridges CEN/TC 229/WG1/TG14 BRIDGE ELEMENTS prEN 13369 1999 Common rules for precast concrete products.

[3.1] [3.2]

Rossner, Brücken aus Spannbeton-Fertigteilen, 1988, Ernst & Sohn, Berlin Vorläufige Richtlinien für Straßen- und Wegebrücken aus Spann- und Stahlbeton-Fertigteilen (R FT-Brücken) 1979, Forschungsgesellschaft für das Straßenwesen

[4.1]

Bridge Design for Durability. A TRL Publication Incorporating Methods of Achieving Continuity in Composite Concrete Bridge Decks by Brian P. Pritchard. Prestressed Beam Integral Bridges by Dr. Edmund C. Hambly and Bruce Nicholson (PCA/U.K. publication) Continuity of Bridge Decks using Precast Beams by Dr H. P. J. Taylor (Chairman PCA/U.K.) Serviceability Limit State Aspects of Continuous Bridges using Precast Concrete Beams by L. A. Clarke and I. Sugie (PCA/U.K. publication)

[4.2] [4.3] [4.4] [5.1]

fib-Guide to good practice (bulletin No. 9, Guidance to good bridge design, July 2000), chapter 1.4.2.

[6.1]

RILEM Report 14 Durability Design of Concrete Structures Edited by A. Sarja and E. Vesikari 1996 fib-bulletin 3 Structural Concrete Textbook on Behaviour, Design and Performance Volume 3 EUROCODE 8-2 DAN Association Française de Génie Parasismique (AFPS) guide, 1992 Ponts Courants en Zone Sismique - Guide de Conception – January 2000, SETRA and SNCF PS92 seismic engineering regulations Association Française de Génie Parasismique (AFPS) recommendations, 1990

[6.2] [6.3] [6.4] [6.5] [6.6] [6.7]

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